A typical submersible well pump assembly of the type concerned herein has an electrical motor that is connected to a centrifugal pump by a seal section. The motor is filled with a dielectric lubricant for lubricating bearings within. The seal section has a pressure equalizing device, which may be a bladder or a U-tube arrangement, for equalizing the pressure of the lubricant in the motor with the well bore fluid on the exterior.
The seal section has a shaft that couples to the motor shaft on one end and to the pump shaft on the other end. A mechanical seal between the housing of the seal section and the shaft reduces encroachment of well bore fluid into contact with the lubricant in the seal section and motor. The mechanical seal typically has rigid faces that are biased toward and rotated in sliding engagement with each other. The rotating component of the mechanical seal is attached to the shaft for rotation therewith. The stationary component is attached to the housing. The sliding seal faces are exposed to well bore fluid on one side and lubricant on the other side. Mechanical seal faces of this nature must remain wet in order to work, therefore they are designed to leak small amounts. Even though the leakage rate is very small, well fluid will ingress into the motor over time and may cause a failure.
One way to prolong well fluid entry into the motor is to provide multiple seal sections, each of which contains at least one mechanical seal. This arrangement adds cost and additional length to the equipment.